1. Field of the Invention
The present invention relates to a receiving device of a code division multiple access (CDMA) system, and more particularly to a multi-path searching system and method.
2. Description of the Related Art
Code Division Multiple Access (CDMA) is a method for transmitting simultaneous signals over a shared portion of the spectrum. The foremost application of CDMA is the digital cellular phone technology from QUALCOMM that operates in the 800 MHz band and 1.9 GHz PCS band, for example.
In a CDMA system, it is important to eliminate inter-chip interference (ICI) produced by a multi-path fading of a self signal. It is also important to reduce multiple access interference (MAI) in order to increase capacity of the system and improve call quality. A rake receiver is generally used in order to reduce the ICI. A beam forming method using an array antenna is used in order to reduce the MAI.
A smart antenna system receives only a signal transmitted from a desired direction, and restrains other signals transmitted from other directions. This selective reception scheme reduces the MAI and thereby increases a channel capacity of a base station. The smart antenna system controls gains and phases of signals received in each antenna device by using a plurality of arrangement antenna devices.
The smart antenna is divided into a multi-beam beam forming method for fixing a pattern of an antenna and an adaptive-beam beam forming method for changing a pattern of an antenna according to time and a communication environment. The multi-beam beam forming method is also called as a fixed-beam beam forming method.
In a typical receiver of a DS-CDMA communication system, a rake receiver is associated with one antenna. The rake receiver allocates fingers to a path of a signal having energy more than a threshold among the searched signals. At this time, in order to allocate the fingers to the path of each signal, a code timing of a corresponding path is provided. In order to synchronize the code timing, the multipath searcher is used.
The conventional multi-path searcher performs a dispreading for a search window size in order to synchronize the code timing, and gets energy of the dispread signals. Then, the multi-path searcher compares a signal Zmax having the largest energy value with a threshold. Herein, if the energy of the signal Zmax is greater than the threshold, the signal Zmax is set as a decision variable of the multi-path searcher. Said processes are repeated whenever slot data is received.
In order to reduce the ICI, an antenna diversity method was applied to the rake receiver in the conventional art. The rake receiver using the antenna diversity method has a high possibility to be adopted in WCDMA in the future. However, the antenna diversity method requires setting a predetermined distance between antennas. Further, the antenna diversity method cannot effectively reduce the MAI in a communication environment having many users.